The present invention relates to an apparatus for driving an objective lens for use in recording/reproducing information to/from a data plane of an optical disk by using light in an optical disk system.
In general, an optical disk system is optically designed so as to minimize aberration at a convergent spot when laser light output from an optical pickup and converged by an objective lens enters vertically to a disk plane. However, a phenomenon of angular aberration between the input beam and the disk plane (hereinafter referred to as a skew) occurs in recording/reproducing signals in an actual optical disk system due to the deflection of a shaft of a turn table during its rotation, the warpage of the optical disk itself and the like.
The higher the density of the signals to be recorded on the optical disk, the greater the adverse effect of the commatic aberration caused by the skew brought about in reading/writing the signals is, so that it is necessary to reduce the disk skew or to correct optically by any means in densifying the optical disk.
For the optical disk system so far, e.g. an optical disk system using a disk having a large diameter such as a laser disk which causes much disk skew, a mechanism as shown in FIG. 26 has been proposed. It is a system of inputting laser light vertically to a disk plane 1000 by rotating an optical pickup 1113 by an angle .theta. centering on an axis of rotation 1016 on a base 1115 of the optical pickup provided slidably on a mechanical deck 1114 when a disk skew angle .theta. is brought about.
However, this system consumes a large power because it is necessary to tilt the large optical pickup 1113 and it has been difficult to correct an AC component of the skew (timewise fluctuation caused by the warpage of the optical disk per each turn) synchronized with the rotation of the laser disk because it is not easy to tilt it at high speed.
With regard to such a problem, there has been proposed a method of tilting only an objective lens 1002 and a prism/mirror group not shown so that an optical axis of the objective lens 1002 is always vertical to a disk plane as shown in FIG. 27 (Japanese Patent Laid-Open No. Hei. 5-6555). This system requires less power in controlling the skew because a movable part 1190 comprises the objective lens 1002, a prism, a mirror and a case for holding them and allows the AC component of the disk skew to be corrected because it allows a fast operation.
However, in tilting the movable part 1190 to correct the skew, the movable part 1190 causes a momental force Mrs of rotational movement centering on an intersection of the optical axis 1117 and the anchoring height of a leaf spring 1118 as a center of rotation EZ as shown in FIG. 28A in a low frequency band (when lower than a primary resonance frequency ftr in the tracking direction).
In a high frequency band (when higher than the primary resonance frequency ftr in the tracking direction) on the other hand, it causes a momental force Mrs of rotational movement centering on of the center of gravity G of the movable part 1190 as shown in FIG. 28B. Therefore, when the movable part 1190 is tiled to correct a radial skew, the principal point O of the objective lens 1002 moves in the radial direction of the disk as indicated by an arrow AR in either cases of FIGS. 28A and 28B and the move of the principal point O turns out be disturbance to the tracking servo, so that it requires a tracking servo gain more than a normal case of using a biaxial actuator.
Further, because the movable part 1190 causes a momental force Mrs of rotational movement around the intersection EZ of the anchoring position of the leaf spring 1118 and the optical axis 1117 during a tracking operation in the low frequency band as shown in FIG. 29, it gives disturbance to the radial skew servo.
Accordingly, the radial skew servo and the tracking servo exert the disturbance on each other, causing the control to be unstable.
When the objective lens 1002 is tilted to correct the skew, the tilting operation for scrutinizing the skew may become disturbance to the focusing servo and tracking servo of the objective lens 1002 due to a reason of an assembly error in fabricating the apparatus for driving the objective lens or of size-wise requirement on the apparatus for driving the objective lens. FIGS. 35A through 35C show such disturbance for example.
Consider a case of starting an operation when the data recording plane of the optical disk D and the objective lens 1002 are far from an ideal relative angle in the prior art apparatus for driving the objective lens 1002 shown in FIG. 27, assuming that the principal point 1195 of the objective lens is located at the position higher than the center of gravity 1191 of the movable part 1190 as shown in FIGS. 35A through 35C. That is, when the objective lens 1002 is tilted in the radial skew direction RSD shown in FIGS. 27 and 35B with respect to the data recording plane of the optical disk D, a skew sensor mounted in the movable part 1190 detects an angle .theta. as a radial skew error signal Ers in many cases during the radial skew servo. FIGS. 35A and 35B show this angle .theta..
In such a state, the radial skew servo is started as shown in FIG. 35B in order to reduce the relative angle between the objective lens 1002 and the data recording plane of the optical disk D. That is, the movable part 1190 causes the momental force Frs by feeding a drive current Irs to the focusing coils 1130.
Thereby, a backward thrust is applied to the movable part 1190 and the objective lens 1002 and the position of the principal point 1195 of the objective lens 1002 moves by .DELTA.Xrs as shown in FIG. 35C when driven in a low frequency. Accordingly, because the position of the principal point 1195 of the objective lens 1002 moves suddenly, disturbance is given to the tracking servo in the tracking direction TRD (radial direction of the optical disk D), thus causing a problem that the tracking servo is undone.
When a current which generates a backward thrust is supplied to the focusing coils 1130 in operating the tangential skew servo along the tangential skew direction TSD in FIG. 27 as shown in FIG. 36, the center of gravity of the movable part 1190 tilts centering on the center of tilt DC which is located on the left from the principal point 1195 of the objective lens 1002 when driven in a low frequency. Thereby, the principal point 1195 of the objective lens 1002 moves along the focusing direction FCS by .DELTA.Xts, thus giving disturbance to the focusing servo.
While the radial skew servo and the tangential skew servo of the objective lens 1002 act slightly as the disturbance on the focusing servo and the tracking servo as described above, there has been a problem that the focusing servo and the tracking servo are undone because a large disturbance is applied suddenly to the focusing servo and the tracking servo when the above-mentioned radial skew servo and the tangential skew servo operation are started in the state when the skew servo error signal which is output by the skew sensor is remarkably large, i.e. when the optical axis of the objective lens 1002 and the data recording plane of the optical disk D are far from being vertical each other.
Further, because the apparatus of Japanese Patent Laid-Open No. Hei. 5-6555 is provided with the light source and optical system for detecting a skew, beside a laser system for reading/writing signals, it hampers in providing a small and low cost pickup. Still more, there has been a problem that the driving sensitivity of the actuator drops because the movable part 1190 shown in FIG. 27 is supported by one cylindrical leaf spring 1118.
Because the movable part 1190 is supported by the thin leaf spring 1118, the leaf spring part may cause plastic deformation, damaging the performance of the actuator, when a strong shock is added from the outside.
Accordingly, it is an object of the present invention to provide an apparatus for driving the objective lens of an optical disk system which allows the deviation of the optical axis of the objective lens and the optical disk from an adequate angle therebetween to be easily corrected.
It is another object of the present invention to provide an apparatus for driving the objective lens of an optical disk system which allows data to be recorded/reproduced reliably on/from a high density optical disk by leading in a skew servo stably even when the skew servo is started in a state when the optical axis of the objective lens and the optical disk plane are far from being vertical each other.